Vertical Takeoff and Landing (VTOL) aircraft have long been considered desirable because of their ability to hover in flight and transition in and out of flight without a runway, in addition to flying in a horizontal direction. The aircraft's lift unit or units have propulsors (e.g., rotor, tiltable jet engines) that develop an aggregate aerial motive force. This aerial motive force can be viewed as the combination of a vertical (i.e., countering gravity) and horizontal (i.e., parallel to ground) vector passing through a single point herein called the “center of lift.” For a VTOL aircraft to be stable and controllable in hover or vertical flight, the vertical vector of its aerial motive force must pass through its center of mass.
Conventional single-rotor helicopters satisfy this requirement by having their center of mass directly below the rotor. (The number of rotors is typically considered the number of rotor axes, irrespective of whether a given “rotor” contains a single set of blades or a pair of counter-rotating sets.) However, that configuration prevents such an aircraft from tilting its rotor for axial flow in horizontal flight with lift developed by a fixed wing. Instead, it must rely on the rotor's own inefficient lift in edgewise airflow, with only enough rotor clearance available for a slight tilt to develop some horizontal airspeed.
As a compromise, aircraft have been developed that include tiltable rotors on opposite wingtips. This configuration has significant drawbacks, perhaps primarily that the prospect of blade interference with a centerline fuselage limits the diameter of paired co-planar rotors to less than half that of a comparable single rotor. The use of paired smaller diameter rotors hurts efficiency, resulting in a hovering propulsive force that is less than 70% of what a single rotor would produce for comparable engine power, but with over 40% greater downwash velocity.
Accordingly, it would be desirable to have a VTOL aircraft that could employ a single rotor for stable vertical flight and hover as well as efficient axial airflow in horizontal flight with lift provided by a fixed wing. It would also be desirable to have a VTOL aircraft, regardless of the type of lift unit employed, with improved control over transition between horizontal flight and vertical or hovering flight.